For example, when a vessel, in particular, a cargo vessel is not loaded with cargo, measures are taken to stabilize a vessel body by loading a ballast tank provided in the vessel with seawater or the like for lowering the center of gravity of the vessel. Since ballast water is discharged outside a vessel at a port of call when loading cargo, an ocean-going vessel might invite a problem that aquatic life such as marine microbes included in ballast water comes and goes between many countries to adversely affect an ecosystem as introduced species.
In recent years, for solving such a problem of ballast water, ballast water discharge is internationally regulated. As a treatment method meeting the regulation, a ballast water treatment system has been proposed. Steps conducted in the ballast water treatment system include a filtering step of capturing foreign matters such as zooplankton, phytoplankton, and suspended particles included in raw water (seawater), a sterilization step of sterilizing plankton, and the like.